1. Field
The present invention relates to an image display device, and more particularly to an optical-scanning-type image display device which displays an image by performing two-dimensional scanning of an image forming light radiated from a light source.
2. Description of the Related Art
Conventionally, there has been known an optical-scanning-type image display device which displays an image by performing two-dimensional scanning of an image forming light which is generated based on an image signal.
As such an image display device, there has been known an image display device which includes a light source part having a light source which radiates a light having intensity corresponding to a supplied electric current, an optical scanning part which performs two-dimensional scanning of the light radiated from the light source of the light source part, and a drive control part which sequentially supplies the electric current of magnitude corresponding to an image signal to the light source when a scanning position taken in the scanning operation performed by the optical scanning part falls within an effective scanning range (see JP-A-2004-9492, for example).
When a semiconductor laser diode or the like, for example, is used as the light source of the light source part, the light source hardly emits light when a current value is equal to or below a threshold current value. Accordingly, a DC bias current having the threshold current value is supplied to the semiconductor laser element which constitutes the light source, and a pulse current on which an electric current corresponding to an image signal is superposed is made to flow in the DC bias current thus allowing the semiconductor laser element to emit a light.
However, the threshold current value of the light source is changed attributed to heat generated when the light source of the light source part radiates the light, a change of ambient temperature or the like. When the threshold current value of the light source is changed in this manner, the current-light emission quantity characteristic of the light source is changed thus giving rise to a drawback that the brightness of a display image becomes unstable.
Accordingly, as a technique which keeps an optical output of the light source part constant, JP-A-7-147446 discloses a following technique. That is, in an optical fiber module, a light source part includes a light source and a light detecting part, a current value supplied to the light source is changed at two points or more, an optical output of the light source is detected by the light detecting part of the light source part, a current-light emission quantity characteristic (a threshold current value or quantum efficiency of a light source, for example) of the light source is detected based on a result of the detection thus setting a bias current supplied to the light source and supplying a pulse current corresponding to the current-light emission quantity characteristic of the light source to the light source.